Compared with traditional substrates for circuit boards, ceramic substrates are excellent in heat dissipation, temperature resistance, reliability, and capable of being made into a thin, small-sized circuit board, and thus they are suitable for high-power chips or dies in electronic products. Both DBC (direct bounded copper) and DBA (direct bounded aluminum) substrates, made of aluminum oxide (Al2O3), are the most commonly used ceramic substrates, wherein the thickness of copper or aluminum layer is generally between 200 and 300 micrometers; when the thickness of copper or aluminum layer is greater than 300 micrometers, because the substrates are easy to suffer interfacial breaks.
With increasing power of chips or dies, the electrical circuit, including pads, traces and conductive layers, on a ceramic substrate need to be thickened. Generally, there is a significant difference between the thermal expansion coefficients of a metal layer and a ceramic substrate. For example, the linear thermal expansion coefficients of copper and aluminum are about 16.5 and 23 ppm/K (at 20 degree C.) respectively, while the liner thermal expansion coefficients of aluminum oxide, aluminum nitride, silicon nitride are about 7, 4.5, 3.5 ppm/K (at 20 degree C.) respectively. The significant difference of thermal expansion coefficients often causes thermal stress at the interface of the circuit layer and the substrate. Under these circumstances, the circuit board easily suffers interfacial breaks, bending or deformation.
To alleviate the problem of existing circuit boards, a technical solution is provided in the present invention, which employs at least one raised or thickened pad of a metal circuit layer on a ceramic substrate for installing a high-power chip, so that most of the temperature difference between the chip and the ceramic substrate is applied across the thickened pad, so that the temperature difference per unit height can be reduced, and thus the thermal stress occurring at the interface between the metal circuit layer and the substrate can be mitigated. Also, the pad can be configured with various thicknesses according to customers' demands.